The reception of a wanted RF broadcast transmitter signal may be disturbed or otherwise deteriorated by various phenomena, such as multipath reception and/or adjacent channel interferences. In general, multipath reception is caused by signal reflections at and/or against environmental physical obstacles such as mountains, trees, buildings, fences and the like. Due to such signal reflections a RF broadcast signal may arrive at a certain reception location through various different signal paths which result in different amplitude and phase conditions. The summation of these multipath signals at the antenna of the receiver results in unpredictable signal amplitude and/or phase distortions. These conditions often effectuate partial or complete cancellation of the useful RF reception signal. These signal cancellations, hereinafter also being referred to as signal dips, strongly depend on the RF carrier frequency of the received RF broadcasting signal and on the location of reception.
The signal dips severely deteriorate the wanted RF broadcasting signal and therefore also the overall signal reception quality. However, a relatively small shift in the antenna position may strongly improve signal reception quality. This solution is used in so-called antenna diversity receivers used with mobile FM receivers to avoid reception of multipath distorted RF signals. Such antenna diversity receivers are provided with two or more mutually spaced apart antennas coupled to a RF input of a receiver. Only the antenna having best local receiving conditions with respect to the other antenna(s) is actually connected to the RF receiver input. This antenna is hereinafter referred to as an actual antenna and is effective in the reception and supply of the wanted RF broadcasting signal to the receiver as long as the multipath distortion at the actual antenna remains smaller than a certain predetermined multipath threshold level. As soon as the received multipath distortion exceeds the predetermined multipath threshold level, a change of the RF signal supply to the receiver from the actual antenna to another antenna positioned at a location with better receiving conditions, is initiated. The receiver is thus continuously optimized for minimum multipath reception.
However, inherent to the antenna diversity feature are short interruptions in the RF signal supply to the receiver during the antenna change over or switching action. Due to the delay between the occurrence of an actual multipath caused signal dip and the detection thereof, the RF signal interruptions may be detected as being caused by multipath effects, and may initiate a subsequent false antenna switching action. The false antenna switching action may in its turn be detected as a multipath originated signal dip initiating a further false antenna switching action with an oscillating effect as a result. To reduce the risk of oscillating antenna switching actions, known antenna diversity receivers using Philips' TEA 6101 model antenna diversity integrated circuit are provided with means to disable any switching action following a preceding switching action within a certain predetermined fixed time period.
This known measure however, is not effective in receivers with dynamic IF selectivity, such as receivers with adjacent channel suppression. As mentioned above, adjacent channel interferences are another important source of signal distortion and are usually caused by FM radio broadcast signals modulated on a carrier positioned in frequency adjacent to the carrier frequency of a wanted FM radio broadcast signal. Due to peak values in the FM modulation signal, these adjacent channel FM radio signals may temporarily exceed the allocated channel bandwidth breaking through into the frequency range of the wanted FM radio broadcast signal. Adjacent channel suppression receivers suppress such adjacent channel interferences by varying the bandwidth of the variable bandwidth intermediate frequency (IF) selector circuits dependent on the deviation of the adjacent channel signal within the frequency range of the wanted FM radio broadcast signal. The larger the deviation, the smaller the bandwidth of the variable bandwidth intermediate frequency (IF) selector circuit. The dynamically varying IF selectivity thus effectuates a suppression of the adjacent channel interferences.
There is thus a need for a receiver which combines the benefits of the antenna diversity feature with those of the dynamic IF selectivity feature while preventing unwanted effects from occurring. There is a further need for an improved performance antenna diversity receiver.